The present invention relates to an information write device and an information read device, for use with information recording media such as a CD-R, DVD-R, or DVD-RW. More particularly, it relates to an information write device and an information read device which are provided with quick access to an information recording medium upon reading and writing.
As is well known, read-only (reproduction-dedicated) CD-ROMs have become widespread, followed by the development of information recording media such as a CD-R and DVD-R (onto which records can be appended) and a rewritable DVD-RW.
These information recording media such as an information writable (recordable) CD-R, DVD-R, and DVD-RW comprise grooves G and guide lands L, which are provided with a predetermined width, arranged adjacent to each other, and formed as spiral Itracks. The media are adapted to allow contents information such as video and audio information to be optically recorded onto the aforementioned grooves G.
On the other hand, information write devices and information read devices which employ these information recording media are provided with an optical pickup (not shown) for radiating the grooves C with a write or read light spot Pc as illustrated schematically in FIG. 17. Upon writing or reading information, the optical pickup is servo-controlled for tracking to allow the light spot Pc to be positioned on and linearly scan over the grooves G.
A tracking error signal is employed for carrying out the tracking servo control. A pair of optical detectors are arranged in parallel to each other in a radial direction θr (perpendicular to the direction of linear scan θs). An information recording medium is radiated with the light spot Pc to produce a reflected beam of light, which is in turn received separately by each of the optical detectors. A difference between the detection signals detected by each of the optical detectors is also determined to generate a tracking error signal. Then, the optical pickup is servo-controlled so that the tracking error signal has a voltage of 0 volt, thereby positioning the light spot Pc on the grooves G.
More specifically, this is schematically illustrated in FIG. 18. The optical pickup is provided with an objective lens OBL for generating the light spot Pc. The farther the objective lens OBL is positioned apart from the center r1 of a groove G1 toward the right (in the portion illustrated as range B), the greater the amplitude of the tracking error signal STE becomes in plus voltages. On the other hand, the farther the objective lens OBL is positioned apart from the center r1 of a groove G1 toward the left (in the portion illustrated as range A), the greater the amplitude of the tracking error signal STE becomes in minus voltages.
When the tracking error signal STE swings to a plus or minus voltage as such, servo control is carried out such that a predetermined actuator is initiated to displace the objective lens OBL toward the center r1 to make those voltages 0 volt, thereby positioning the light spot Pc on the center r1 of the groove G1.
That is, the tracking servo control is carried out such that the tracking error signal STE has a value of 0 volt, with the ranges A and B taken as a focus retract region in a half cycle in which the tracking error signal STE takes on the maximum plus and minus amplitude.
In addition, these CD-R, DVD-R, and DVD-R have a good function as an information recording medium which enables random access. To implement the random access, the information write device and the information read device release temporarily the aforementioned tracking servo control to move the optical pickup to the vicinity of the target position in the radial direction θr under this condition and thereafter restart the tracking servo control to position the optical pickup at the original target position.
For example, suppose that the objective lens OBL oriented to the aforementioned groove G1 is moved to the center (the target position) r2 of the groove G2. In this case, the pickup is moved at once to a proximal position rf near the center r2 with the tracking servo control being released. However, the proximal position rf is set to be within the lock range in the groove G2. Then, the tracking servo control is restarted to thereby position the light spot Pc at the center r2 on the groove G2 so that the tracking error signal STE has a voltage of 0 volt within the lock range indicated by the region C and D.
At this point, the circuit as shown in FIG. 19 was employed to determine whether the pickup reached the aforementioned proximal position rf. That is, the aforementioned optical detector detects the reflected beams of light generated when the main light spot Pc crosses the groove G and the land L alternately, and the RF signal SRF generated from the detected signal as shown in FIG. 20(a) is supplied to an envelope detector 100, thereby generating an envelope signal STB as shown in FIG. 20(b). Furthermore, the envelope signal STB is supplied to a comparator 102 to be compared with a predetermined threshold THD, thereby generating a binary encoded contrast signal SRC as shown in FIG. 20(c). Then, the position to which the pickup is to be moved is determined in accordance with a count Value Scn obtained by counting the contrast signal SRC with a counter 104.
However, when a non-recorded area where no information has been recorded is randomly accessed, the aforementioned prior-art information write device and information read device were not able to obtain the contrast signal SRC with accuracy. Thus, in some cases, it was difficult to move the pickup quickly to the target position with accuracy.
That is, the groove G of the non-recorded area where no pits have been formed yet made a difference in intensity smaller between the reflected beam of light from the groove C and that from the land L. Thus, in some cases, it was not possible to obtain the contrast signal SRC which can clearly distinguish the groove G from the land L.
In particular, when warping or strain exists in the non-recorded area of the information recording medium, a noise component of low frequency caused by the warping or the strain was to be superimposed on the RF signal SRF and the envelope signal STB. Consider a case where the noise component of low frequency was greater in amplitude than the reflected signal component from the groove G and the land L. When the envelope signal STB was supplied to the comparator 102 to be compared with the threshold THD, an adverse effect would occur such that the aforementioned noise component was accidentally determined to be the groove G or the land L. Thus, in some cases, it was impossible to obtain the contrast signal SRC which can clearly distinguish the groove G from the land L. Consequently, in some cases, it was impossible to move the pickup quickly to the target position with accuracy.